Nonlinear seismic assessment of eight-storey reinforced concrete building according to Eurocode EN

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1 Nonlinear seismic assessment of eight-storey reinforced concrete building according to Eurocode EN 99- V. Lekidis ), S. Anagnostopoulos ), D. Baros ), Ch. Karakostas ), Th. Salonikios ) and T. Makarios ) ), ), ), ), Earthquake Planning & Protection Organization, ITSAK, Thessaloniki, Greece ), ) Department of Civil Engineering, University of Patras, Greece ) lekidis@itsak.gr ABSTRACT In the present article, the nonlinear seismic assessment of an irregular -storey reinforced concrete building according to Eurocode EN 99- () and the recent Greek Code for Retrofitting of Reinforced Concrete Buildings is presented. The assessment was prompted from safety concerns generated by a planned removal of all the infill walls in the first story and their replacement by light-weight partitions. What makes this study interesting is: (a) the fact that the building had been repaired and strengthened after an M=. damaging earthquake in 9, and (b) the availability of the strong motion records from that earthquake from an instrument at the building basement. The seismic capacity assessment of the building is based on nonlinear dynamic and static, taking into account the increased seismic safety levels imposed by the current Codes. Modeling issues pertaining to the limit states of reinforced concrete members, e.g. member end rotations at yield, full plastification and failure, effective member stiffness for nonlinear analyses, are discussed and the selections made for this work are presented. Infill walls are modeled using simple strut members working only in compression. The required properties were taken from available drawings, but were also verified by in situ measurements. The study indicated that removal of the first story infill walls had no consequences on the seismic safety of the building but also showed that if the building is hit by a current design level earthquake, some damage might be expected in the upper floors at one side of the building.. INTRODUCTION The investigation reported herein was carried out for the administration building of the Messinia prefecture, an -story structure in the city of Kalamata in southern Greece (Fig. ). This building has an elongated rectangular shape, with dimensions of its framing plan L=.m and B=,m. In the middle of the building there is an expansion joint perpendicular to the long direction, acting also as a seismic separation gap. Since the building is symmetric with respect to the axis of this joint, only the left ) Research Director

2 half of it was considered (Fig. ) This building was retrofitted after been damaged in an M=. earthquake that struck the city in 9 (Anagnostopoulos et al, 9).The need for assessment of its seismic capacity arose when plans were announced to remove all the masonry infill walls in the first story and replace them with light partitions. There were certain concerns by various employees for this change, based on the argument that the building would be weakened. This prompted the investigation of the seismic capacity of the building as is and the effects, if any, of removing the first story partition walls. Thus two models of the building were created and investigated, with and without infill walls in the first story, while everything else remains the same. Fig. The eight-story R/C building in Kalamata Fig. Layout of the eight-story r/c building (dimensions in meters).

3 Correct modeling of a structure is always a prerequisite for correct numerical results. Such modeling must include members and elements that may not always be characterized as structural, e.g. brick infill walls, which, however, possess significant initial stiffness and strength. When nonlinear analyses are used for capacity assessment, modeling requirements are substantially more complicated and difficult compared to those for elastic analyses, given that one must calculate () Moment- Chord Rotations ( M diagram) at each critical end of each structural member, () available cyclic shear strength of each structural member, which determines the expected mode of failure, ductile or brittle, of the member, () effective member flexural stiffness, () moment-axial force interaction diagrams for beam-column elements, and () initial stiffness and strength of the masonry infill walls. The first four of these items are fully covered by Eurocode EN 99- () and the recent Greek Code for Retrofitting of Reinforced Concrete Buildings (KANEPE, ), while for the infill brick walls information from the literature has been used (Makarios, ). (a) (b) (c) (d) (e) Fig. (a,b) Magnetic tests for detection of steel bar location (c,d) Concrete strength measurement (e) slab coring (g) Detection of concrete carbonation (g)

4 . DATA AND MODEL OF THE EIGHT-STOREY R/C BUILDING In order to verify the design assumptions and available drawings, some coming from the original design and some from the retrofitting phase, a series of non-destructive in situ tests was carried out (Fig. ). The results indicated a characteristic concrete strength of. MPa, mean strength. MPa and Young s modulus E =. GPa. The mean steel strength is MPa and the maximum strength 9 MPa, reached at a strain of % plus. The steel strain at failure was found to be %. The elastic and inelastic analyses of the building were carried out with the finite element program SAP. The D geometric model of the building is shown in Fig.. masses m and mass moments of inertia J m were calculated analytically and are listed in Table for each floor. In the same Table, the floor elevations z from the ground reference level are also given. The idealization of the masonry infill walls is made with equivalent diagonal truss elements, as shown in Fig. for the building model with the first story infills removed. Fig. D model of the eight-storey r/c building Table elevation z, masses m (tons) and floor torsional mass-moments of inertia J m (tons. m ) Fl. z m J m....

5 Fig. Infill wall modeling with first story infills removed: The two internal frames in the longitudinal direction Table The first three periods of the eight-storey building Τ Τ Τ Model : Building as is.9 s. s. s Model : First story infills removed.9 s.9 s.9 s Fig. Maximum floor displacements by response spectrum analysis Comparison of the two models (floor elevations in Table )

6 . RESPONSE SPECTRUM ANALYSIS The response spectrum analysis was carried out for ground acceleration A=.g, (g = acceleration of gravity) applicable to the seismic zone for Kalamata, as per the current code for new buildings, and the elastic design spectrum of Eurocode for soil category D. Table shows the lowest periods of the two models, which are, as expected, almost identical. This indicates a practically zero influence of the first story infills on the building s stiffness. The same conclusion is reached by looking at the maximum floor displacement profiles of the two models, computed by response spectrum analysis for the EC- design spectrum (Fig. ). Moreover, comparing the elastic bending moment of the columns for the two aforementioned models, all the differences are below %. Thus, one can conclude that the elastic building response is practically unaffected by the removal of the first story infills. The same analysis indicates that bending moments in columns C (on ground floor), C C C C C (on the sixth floor), and C in all levels, as well as in several beams exceed the corresponding bending strengths, which points to the need for further investigation using inelastic analyses. Fig. Inelastic beam modeling for the SAP program

7 . NONLINEAR SIMULATION OF STRUCTURAL R/C MEMBERS Modelling of the concrete beams and columns was carried out as required by the non-linear computer program SAP and detailed in another paper (SAP,, Makarios, ). The so called one-component, plastic hinge model is used, where each prismatic flexural member is idealized with three sub-elements: the elastic prismatic beam or beam column and two nonlinear springs of zero length attached at the two ends of the elastic element. In order to find the characteristics of a plastic hinge, it is assumed that each member (beam/column) deforms in antisymmetric L bending. The required shear length, s,i, is determined from the point of contra-flexure (Fig. ), while the Moment-Chord Slope Rotation ( M ) diagrams of the non-linear springs are determined on the basis of the element properties (concrete section, and reinforcement properties) using a special purpose program based on the fiber model (XTRACT, ) In order to calculate the M used: (a) one as proposed by Eurocode EN-99. (Annex A: sections from A... until A... and derived from a large amount of experimental data (Panagiotakos & diagram of a plastic hinge, two methods can be Fardis ). (b) using a moment-curvature ( M - ) diagram for numerical computations by means of a fiber model of the member, as implemented in various computer codes ( XTRACT, ; Section-Designer /SAPv, ) This requires L a suitable length p for each plastic hinge (Fig.b). In these calculations the limiting yield, plastic and ultimate rotations θ are computed as follows where y, u θ y =φ. y L s / θ p =(φ u -φ. y) L p θ u = θ y + θ p L s = M V are the yield and ultimate curvature of the end section, respectively, Ls L the shear length and p the plastification length with assigned values based on experimental results (e.g. for a beam taken equal to its depth). Additional information on the plastification length for different concrete structural members can be found in the literature (e.g. Panagiotakos & Fardis,, Salonikios;, Paulay & Priestley 99). Μ is the elastic flexural moment and V the corresponding shear force at the L member end due to earthquake. For tall-walls, the shear length s can be calculated as the distance, in elevation, of the zero-moment point to the basefor a set of lateral, earthquake static floor forces. For the present paper M-θ diagrams were calculated using the program XTRACT. Note that our calculations did not consider sources of inelastic behavior such as slippage of reinforcement or opened cracks with yielding steel bars. Masonry infill walls were modelled for our analysis using diagonal struts working only in compression (Fig. ) in accordance with the new Greek code for retrofitting existing

8 concrete structures (KANEPE ). More details can be found in Makarios,. Fig. Stress-strain diagram for diagonal compressed inelastic bar (Masonry infill walls). NON LINEAR DYNAMIC RESPONSE HISTORY ANALYSES (NLDA) For the nonlinear analyses, the flexural stiffness of the various beams were based on secant stiffness at yield, which according to modern codes for retrofitting existing structures, e.g. KANEPE, EC-, is only a small fraction of the flexural stiffness EI for elastic analyses. For an idea of the resulting differences in the overall stiffness of the building, the periods corresponding to secant member stiffness were computed and listed in Table. Comparison with the corresponding periods in Table indicates that the inelastic models are % and % softer than the model used for the response spectrum analysis (~ elastic) along the y and x axes respectively. The nonlinear response history analyses (NLDA) were performed using three pairs of semi-artificial motions, compatible with the Design Spectrum of Eurocode EN 99, generated using the method by Karabalis et al, 99. Additionally, the recorded motion at the base of the building from the 9 earthquake was used (Anagnostopoulos et al, 9) for comparison and verification purposes. Fig.9 shows the code design spectrum explained previously, the response spectra of the semi-artificial motions and the response spectra of the two horizontal components of the 9 records at the building basement. Table Periods and effective modal masses for the two models of the building Mode shape With masonry infill walls of the st floor Τ (sec) M* x (%) M* y (%) Τ (sec) M* x (%) M* y (%)

9 Spectral acceleration S a (g) EAK elastic design spectrum Semi-artificial records: Kalamata NE-L (semiartl-x) Kalamata NW-T (semiart-y) Lefkada -L (semiart-x) Lefkada -T (semiart-y) Alkyonides L (semiart-x) Alkyonides T (semiart-y) Natural records: Kalamata NE-L Kalamats NW-T Period T (sec) Fig. 9 Response spectra of the semiartificial motions and of the Kalamata records and elastic acceleration spectra of used accelerograms The response history of the building top (center of mass) along the longitudinal (x) direction for both models under the two component real Kalamata record is shown in Fig.. The influence of the first story infills is negligible. The same conclusion, i.e. negligible influence of the first story infill walls, can be observed in the maximum displacement profiles for the three artificial motion pairs (Fig. ) and in the interstory drifts (Fig. ) shown as percentages of the story heights. Roof displacement, u x with masonry infill walls without masonry infill walls of the st floor 9 Time (sec) Fig. Response history of center of mass at the building top under the action of the real, two component, Kalamata record 9

10 Set Set Set Displacement, u x Set Set Set Displacement, u y With masonry infill walls of the first floor Set Set Set Displacement, u x Set Set Set Displacement, u y of the first floor Fig. Maximum floor displacements of the two models Set Set Set Inter-story drift, 훧 x Inter-story drift, 훧 (% of story height).. Set Set Set With masonry infill walls of the first floor y

11 Set Set Set Inter-story drift, 훧 x Inter-story drift, 훧 (% of story height).. Flor Set Set Set of the first floor y Fig. Interstory drifts (% of story height) for the two models. STATIC PUSHOVER ANALYSIS In addition to the non linear dynamic analyses, static nonlinear (pushover) analyses have also been performed for investigation reasons. The required target displacements were estimated as suggested by the code and for comparison also as the mean peak displacements from the NLDA solutions. The agreement, as it can be seen in Fig. is quite satisfactory. From these pushover curves, as well as from the graphs of corresponding interstory drifts, Fig., it becomes apparent again that the removal of the first story infill walls has practically negligible consequences on the seismic capacity of the building. Base shear (kn) With masonry infill walls: Pushover curve Target displacement? t roof displacement from time-history analyses of the st floor: Pushover curve Target displacement? t roof displacement from time-history analyses Roof displacement, u x (a) With masonry infill walls: Pushover curve Target displacement? t roof displacement from time-history analyses of the st floor: Pushover curve Target displacement? t roof displacement from time-history analyses Roof displacement, u x (b)

12 With masonry infill walls: Pushover curve Target displacement? t roof displacement from time-history analyses With masonry infill walls: Pushover curve Target displacement? t roof displacement from time-history analyses Base shear (kn) of the st floor: Pushover curve Target displacement? t roof displacement from time-history analyses Roof displacement, u y (c) of the st floor: Pushover curve Target displacement? t roof displacement from time-history analyses Roof displacement, u y Fig. Pushover capacity curves of the building along the main directions +X, -X, +Y & -Y (d) With masonry infill walls: of the st floor: With masonry infill walls: of the st floor: Inter-story drift, 훧 x Inter-story drift, 훧 (% of story height) (a) x (b) With masonry infill walls: of the st floor: With masonry infill walls: of the st floor: Inter-story drift, 훧 (% of story height) y Inter-story drift, 훧 (% of story height) (c) Fig. Interstory drifts by pushover analysis (a) along +X, (b) along -X, (c) along +Y & (d) along -Y y (d)

13 . CONCLUSIONS Based on the above and other results not reported herein, the following conclusions have been drawn: (a) Removal of the first story masonry infill walls has practically no effect on the seismic capacity of the building. (b) Under current design standards for new buildings, as specified in Eurocode EN 99- (), some damage is predicted in the beams connecting to the major shear walls of the building. More significant is the damage expected in the upper stories of columns around the seismic separation of the two building units, namely in columns C, C, C and C (Fig.). (c) This suggests that some local strengthening might be required, but not as a result of the planned removal of the first story infill walls. REFERENCES Anagnostopoulos, S., Rinaldis, D., Lekidis, V., Margaris, V., Theodulidis, N. (9). The Kalamata, Greece Earthquake of September, 9. Earthquake Spectra ; (): -. Computers & Structures Inc. SAP Structural Analysis Program. ver., Berkeley, California, USA,. EN 99-. (). Eurocode : Design of structures for earthquake resistance Part : General rules, seismic actions and rules for buildings. European Committee for Standardization, Brussels. EN 99-. (). Eurocode : Design of structures for earthquake resistance Part : Assessment and retrofitting of building. European Committee for Standardization, Brussels. KAN.EPE. (). Hellenic Code of Retrofitting of Reinforced Concrete Buildings. Earthquake Planning & Protection Organization (E.P.P.O). Karabalis DL, Cokkinides GJ, Rizos DC, Mulliken JS, Chen R. (99) An interactive computer code for generation of artificial earthquake records. Computing in Civil Engineering (ASEE) 99; In K. Khozeimeh (ed.) - Makarios T. (). The equivalent non-linear single degree of freedom system of asymmetric multi-storey buildings in seismic static pushover analysis. CHAPTER of Book Earthquake Research and Analysis / Book th. Makarios T. (). Modelling of characteristics of inelastic member of reinforced concrete structures in seismic nonlinear analysis. CHAPTER of Book Focus on Nonlinear Analysis Research, Nova Science Publisher, Inc. April. Panagiotakos T, Fardis M. (). Deformations of reinforced concrete members at yielding and ultimate. ACI Structural Journal, March-April;V.9, No.. Paulay T, Priestley (99). MJN. Seismic Design of Reinforced Concrete and Masonry Building. Willey, New York. Salonikios T. (). Deformation characteristics of r/c walls, with low aspect ratio,

14 subjected to cyclic loads. Fib Symposium: Concrete structures in Seismic Regions ; May, CD-ROM, Paper Number ; Athens. XTRACT. v... (). Cross-sectional X structural Αnalysis of ComponenTs. Imbsen Software System. 99 Business Park Drive, Suite, Sacramento CA 9.